Collapsing a folder causes crashes if Selected in preview is active

[kamilburda]

Plug-in crashes when interacting with the preview.

Steps to reproduce:

1. Add the Selected in preview constraint.
2. Select a layer within a layer group (folder).
3. Collapse the layer group by clicking the expand/collapse button next to the folder icon.

Result: Plug-in crashes and is unresponsive (the error message cannot even be closed).

Workaround: Disable the Selected in preview constraint and collapse the folder.